1. Field of the Invention
The present invention relates to a piezoelectric resonator and, more particularly, to a piezoelectric resonator for high frequencies (100 to 300 MHz) based on a thickness extensional oscillation mode or a thickness shear oscillation mode.
2. Description of the Prior Art
In general, a piezoelectric resonator for high frequencies which relies upon a thickness extensional oscillation mode or a thickness shear oscillation mode, realizes resonance by using fundamental waves by decreasing the thickness of the piezoelectric substrate or by using third harmonics without decreasing the thickness of the piezoelectric substrate.
When the fundamental waves are used, the piezoelectric resonator must have a piezoelectric substrate of which the thickness is precisely decreased. When harmonics are used, the piezoelectric resonator must be so designed as to increase a peak valley value (P/V value) of the tertiary mode while suppressing the fundamental waves. In either case, dispersion in the thickness of the drive electrode must be decreased to be as small as possible since high frequencies are used.
When fundamental waves having such high frequencies as 100 to 300 MHz are used, the piezoelectric substrate must have such a small thickness that it becomes difficult to fabricate a piezoelectric resonator. The resonance frequency of a piezoelectric resonator using the thickness extensional oscillation or the thickness shear oscillation, varies in reverse proportion to the thickness of the piezoelectric substrate. In the case of a fundamental mode and when the piezoelectric substrate is made of a general piezoelectric material, the thickness must be decreased down to several tens of microns or smaller. To suppress the dispersion in the resonance frequency to be smaller than 1%, further, the dispersion in the thickness of the piezoelectric substrate must be suppressed to be not larger than 1%. Besides, the resonance frequency is more seriously affected by the dispersion in the thickness of the drive electrode as the thickness of the piezoelectric substrate decreases.
As described above, an increase in the resonance frequency makes it difficult to fabricate the piezoelectric resonator. When a piezoelectric resonator of a high resonance frequency is to be fabricated, therefore, it is a generally accepted practice to use a harmonic mode such as of the third harmonics.
In an ordinary design, in general, the piezoelectric resonator is more strongly excited with the fundamental waves than with the harmonic waves. To correctly excite the piezoelectric resonator with harmonics, therefore, it is required to increase the impedance peak of harmonics to be greater than that of the fundamental oscillation. It is therefore desired to so design the resonator as to suppress the fundamental mode and to develop third harmonics which are as neat as possible. As the frequency becomes high, however, the absolute value of P/V value of the tertiary mode decreases systematically and, besides, it becomes difficult to decrease the oscillation in the fundamental mode.
In order to solve the above problems, Japanese Unexamined Patent Publication (Kokai) No. 341064/2000 and Japanese Unexamined Patent Publication (Kokai) No. 205076/1999 propose quarts oscillators utilizing a fundamental wave mode in which the base portion of the resonator is formed thick and the oscillation portion is formed thin. FIGS. 15a to 15c illustrate the above piezoelectric resonator in which dents 33a and 33b are formed in both surfaces at the central portions of a square piezoelectric substrate 31 at positions facing each other, a pair of opposing drive electrodes 35a and 35b are formed in the dents 33a and 33b, and electrode wires 37a and 37b are connected to these drive electrodes 35a and 35b. 
This piezoelectric resonator is suppressed from being broken since the piezoelectric substrate 31 is surrounded by a thick outer peripheral portion, but is accompanied by a problem in that it is very difficult to control the thickness of the piezoelectric substrate 31 of a portion where the drive electrodes 35a and 35b are formed maintaining good precision without dispersion. It is further necessary to highly precisely adjust the thickness of the drive electrodes 33a and 33b. The piezoelectric resonator of this structure must be precisely machined relying upon the etching technology inevitably driving up the cost of producing the devices.